R&D of Photovoltaic Thermal (PVT) Systems: an Overview

Received Dec 19, 2017 Revised Jan 18, 2018 Accepted Feb 3, 2018 Photovoltaic thermal (PVT), which is the popular technology for harvesting solar energy, receive solar energy and convert it into electrical and thermal energy simultaneously. In this review, design, heat transfer, energy modelling and performance analysis of PVT systems are presented. Four types of PVT systems base on heat transfer medium; air-based PVT system, water-based PVT system, the combination of water/air-based PVT system, and nanofluid-based PVT system are presented. In addition, major finding on energy and exergy analysis of PVT systems are summarized. Keyword:


INTRODUCTION
The depletion of fossil fuels resources has revived the interest in renewable energy resource utilization [1]- [2]. As a result, various research and development (R&D) activities have been conducted to identify reliable and economically feasible alternative sources of clean energy. The choices include solar, wind, wave, and geothermal energies. Among these energy types, solar energy, which is widely used in heating and cooling applications, is the popular source [3]. Solar energy converts to electric energy using photovoltaic (PV) technology. The cons of this PV cell are declining in efficiency conversion when temperature raised and only responsive to a portion of the solar spectrum. The low efficiency and high cost of PV cell brings the idea of PVT system. It is the integration or hybrid of PV panel with solar thermal collector. The advantage of PVT enhances the electrical energy produce, removes waste heat from PV panel and minimized the usable space. Moreover, solar energy will convert to thermal energy as stored in air or water. PVT builds up from glass cover, solar cell, encapsulated materials and collector attached at the back. In terms of physical structure applied, the module could be classified as flat plate, concentrated and building integrated types. The absorber functions to absorb heat and simultaneously cool down the PV panel. The collected heat will be in the form of fluids or nanofluids.
Recently, energy and exergy analysis for PVT systems were studied base on experimental and theoretical study. A theoretical approach (mathematical model) to predict outlet and PV temperatures of finned PVT air collector system was studied [4]. The overall performance of the PVT system can be evaluated based on the thermodynamic, environmental and economic impacts analysis. Energy-exergyeconomic-environmental analyses for different PVT array systems was studied [5]. In this review, thermodynamic analysis is focused involving Secondary Thermodynamic Law or known as exergy analysis. It has become an essential tool in the system design, analysis, and optimization of thermal systems [6][7][8][9][10][11].

TYPES OF PVT SYSTEMS
PVT is the popular technology of a solar energy technology. PVT system is a device designed to receive solar energy, convert it into electrical and thermal energy, which transfer the thermal energy to the fluid that flows into the collector. Fig. 1A shows a PVT consisting of a PV panel, insulation and frame. Accordingly, PVT consists of one or more cover (glass sheets) or a transparent material placed above an absorbing plate with air flowing around it. One way to enhance the collector's efficiency of PVT system is use heat transfer area through absorber with corrugated surfaces (Figs. 1B), finned absorber (Fig. 1C), and porous media (Fig. 1D). PVT system can be classified into four types base on heat transfer medium; airbased PVT system (Figs. 1A-D), water-based PVT system (Fig. 1E), the combination of water/air-based PVT system, and nanofluid-based PVT system (Fig. 1F). Fig. 2 shows schematic and photograph of combination of water/air-based PVT system.

ENERGY AND EXERGY ANALYSIS OF PVT SYSTEMS
The thermal efficiency of PVT system is a ratio of the useful thermal energy, to the overall incidence irradiations, The heat collected by the flat plate PVT collector can be measured by result of average mass flow rate, heat capacity of flowing medium, and a temperature difference of the medium at the collector inlet, and outlet, [14][15][16][17].
The electrical efficiency of PV, is a function of temperature given by where is the reference efficiency of the PV, is the temperature coefficient ( 0.0045 °C), and is cell temperature and the reference temperature [17].
Exergy is defined as the maximum amount of work that can be produced by a stream of matter, heat, or work as it reaches equilibrium with a reference environment. In the past few decades, exergy analysis has become an essential tool in thermal system design, analysis, and optimization [6][7][8][9][10][11]. If the effects of kinetic and potential energy changes are neglected, then the general exergy balance rate can be expressed in the following rate form: The exergy and exergy efficiency of PVT system is expressed as Exergy analysis may be proposed using the sustainability index (SI). It can be expresed as [18] Ex SI    1 1 Another performance of exergy analysis is improvement potential (IP). It is useful to the efficient analysis of processes or systems. The IP of a process or system is calculated by [19,20]: Recently, Fudholi et al. [21] studied theoretical and experimental of PVT air collector with ▽groove, as shown in Table 1. They reported that PVT energy efficiencies were 31.21-94.24%, and PVT exergy efficiencies of were 12.66-12.91%. The PV and thermal efficiency was 9.87-11.34% and 21.3-82.9% respectively. Several studies on the energy and exergy analysis of PVT systems were reported as shown in Table 2. Salem et al. [22] studied exergy and energy analysis of hybrid PVT system using aluminium cooling plate. They reported that PVT exergy efficiencies of were 11.1-13.5%, and PVT energy efficiencies were 59.3-92%. The PV and thermal efficiency was 17.7-38.4% and 31.6-57.9% respectively. Lari and Sahin [23] reported that PV energy efficiency was 13.2% for PVT nanofluid system. Khanjari et al. [24] reported that PV, thermal and PVT energy efficiencies was 10-13.7%, 55% and 90% respectively, and PVT exergy efficiency was 15%. Tripathi et al. [25] studied exergoeconomic and enviroeconomic analysis base on energy and exergy for PVT concentrating collector. They reported that thermal and PVT energy efficiencies were 40-50% and 45-63%, respectively. Singh et al. [26] studied energy and exergy for active solar still integrated with two hybrid PVT collectors. They reported that thermal and PVT energy efficiencies was 69.06% and 75%, respectively. Energy and exergy analyses for PVT air collectors were studied [27,28]. Hazami et al. [27] and Gholampour and Ameri [28] reported that PVT exergy efficiency was 14.8% and 8.66%, respectively. Exergoeconomic and environmental analyses for PVT mixed mode greenhouse solar dryer was studied [29]. They reported that PVT energy efficiency was 68.5%. Exergoeconomicenviroeconomic--environmental-exergy-energy analyses were studied for active solar distillation system [32] in 2015.

CONCLUSION
Based on the present review, the following conclusions can be drawn; 1) A number of research have been done on PVT systems over the last four decades, exploring aspects such as efficiency enhancements by design development, numerical simulation, prototype design, experimental testing and testing methodologies for PVT systems, 2) The development of PVT system is a very promising area of research. Today, PVT systems using in various applications, such as solar drying, solar cooling, water heating, desalination, and pool heating.